1. Field of the Invention
The present invention is related to the field of flexible plastic light conduits that are doped with laser dyes, and illumination applications of the same. The present invention is specifically related to the field of fluorescent plastic optical fibers formed by a monomer such as methyl methacrylate (MMA) [CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.3 ] that is cross-linked by a co-monomer such as allyl diglycol carbonate (CR-39), the mixture of which is further doped with high quantum efficiency fluorescent dyes, such as those used in dye lasers.
2. Description of the Prior Art
Glass fibers are used typically to transmit light in communication applications, where the fine optical properties of glass are exploited to achieve the lowest possible attenuation of transmitted light signals. Glass fibers, however, have been generally unsuccessful when used for transmitting light for purposes of illumination (except for short distance applications), because of disadvantages such as brittleness, expense, small diameter, and a general inability to pass high levels of illumination (a result caused by their small diameters). Indeed, to increase the light transmission capabilities of commercially available small diameter glass fibers, many of them are bundled together into a single light conduit; however, such bundles are expensive to manufacture, heavy, and the typical maximum diameter of such bundles is limited to approximately 3/4". In addition, the adhesive used in the manufacturing process of the bundles tends to degrade at temperatures generated by light sources that are typically present, such as tungsten halogen or arc lamps.
In fibers used for side illumination applications, it is desired for the light to escape from the core of the fiber through the cladding without a drop-off of illumination intensity along the length of the fiber. Additionally, there should be preferably a large amount of side light for better visibility. Fibers used for illumination should be of large diameter, so that they can be visible from a distance, such as when used for road signs. Such fibers should have good resistance to environmental factors such as heat, cold, sunlight, moisture, etc.
The very characteristic of high optical quality that makes glass fibers desirable for applications in the transmission of light is also the reason that glass fibers are not well suited for illumination applications. If light propagates and emerges as end light, without "loss" through the cladding, then effective side illumination is not achieved.
In side illumination applications, plastic optical fiber light conduits exhibit some marked advantages over their glass counterparts--light weight, low cost, flexibility (lack of brittleness), and large diameters (up to 1" are possible). Plastic optical fibers do not exhibit as good an optical quality as glass; however, a high optical grade quality is not required for such applications and, indeed, plastic optical fibers are finding a wide variety of side illumination applications, from road signs, advertising signs, billboards, and store displays, to illumination in explosive environments.
One current application for flexible optical fibers is to approximate the appearance of neon color tubes. Heretofore the method for approximating this neon "look" has been by illuminating currently available clear fiber with a tungsten halogen or metal halide lamp, and sometimes also utilizing a changing color wheel to provide variations in the color of the pumping light. Such illuminating is described in U.S. Pat. No. 4,704,660 to Robbins, U.S. Pat. No. 5,111,367 to Churchill, U.S. Pat. No. 5,222,793 to Davenport et al., and U.S. Pat. No. 5,321,586 to Hege et al. Such illuminating uses a light source (e.g. a tungsten halogen or metal halide arc lamp) for optical pumping in conjunction with a collection device (usually an elliptic reflector). The reflector focuses the light onto a narrow entry point at the input end of the fiber. A color filter wheel may be positioned between the reflector and this fiber entry point to interpose variously colored filters successively between the fiber and the beam. Where used to color the fiber, such color wheels are usually powered to provide a changing succession of colors using a color wheel (rotation rate of 0.5 to 5 RPM).
To use plastic optical fibers for neon-like illumination, it is desired that (a) a high percentage of the input light "escapes" from the fiber as side light, and that (b) the fiber can accommodate a large amount of input light. Typical clear optical conduits, when used for side lighting applications, transmit about 60% to 90% of the light out of the output end of the fiber. This means that only a small portion (i.e. 10%-40%) of the light energy is transmitted through the walls of the fiber, causing the neon-like effect to appear faint and washed out, without the vibrant "glow" of a true neon gas discharge tube.
To achieve a more neon-like intensity and appearance with these prior art fibers, higher power levels of input light have been attempted. Higher light power levels, however, are typically accompanied by higher power requirements, and resulting higher heat levels and higher operating temperatures, both for the illuminator and for the fiber (at the point where the input light is focused on the input end of the fiber). Such temperatures might reach as high as 150.degree. C. to 250.degree. C. Plastic optical fibers will not survive such temperatures without damage. Typical optically clear thermoplastic polymer fibers tend to degrade at temperatures above 85.degree. C. Further, heat filters can only provide a limited protection to fibers in the best of cases.
In addition, a large size becomes difficult to attain. In order to manufacture plastic optical fibers of a diameter greater than 1/8" out of these thermoplastic polymers, plasticizer may be added to the mixture to make them flexible. The addition of a plasticizer tends to further lower the service temperature of the resulting fiber, making it susceptible to temperature-caused degradation even at temperatures lower than 85.degree. C.
The heating problem associated with using high intensity pumping close to a transmission fiber has been addressed by various illuminator configurations. In U.S. Pat. No. 5,111,367, Churchill discloses an illuminator with means for dissipating heat. The light is not focused, but is allowed to emit in all directions, and therefore the various fibers only collect a small percentage of the total light output of the lamp. Accordingly, the collection efficiency in this scheme is low. The resultant smaller collection angle obtained with this configuration will yield a lower overall temperature at the fiber end; however, much potentially usable light intensity is sacrificed.
The fiber optic illuminator described in U.S. Pat. No. 4,704,660 of Robbins has attempted to cool the apparatus by using a clear transparent fan blade interposed between the light (and heat) source and the entry point to the end of the fiber. The fan provides cooling at the expense of some focusing efficiency of the reflector, because of the diffusing effect of the fan blade passing between the light source and fiber input.
In trying to approximate a neon-like effect, prior art patents have used optically clear fibers and have succeeded in generating only a relatively low intensity glow. Significant obstacles need to be overcome regarding efficient heat removal at the fiber input end, in order to obtain enough power to simulate a true neon-like look with these configurations.
For illumination purposes, then, a significant need exists for a light conduit which has the properties of being able to transmit light effectively while allowing a large portion of light to escape uniformly at high intensity along its length. A need exists for a light conduit that would further be able to be manufactured in larger diameters, be of light weight, flexible, rugged, and capable of operating under high service temperatures and adverse environmental conditions. This conduit should be relatively economical to manufacture and able to transmit light in colors ranging across the visible spectrum bandwidth (from 400 to 750 nm).